CCIE Lab

Unlike Linux’s iptables, Cisco’s filtering via Access Control Lists sometimes has hidden behavior.

Let us test how ACL filtering works using the following topology. We assume that we have Layer 3 connectivity via static routes. We will apply ACLs on the outbound direction of F1/0 on R2 (we want it to be somewhere in the path from R1 to R3)

With no ACLs applied anywhere, all traffic will flow.

R1#ping 3.3.3.3 source 1.1.1.1
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent

Let’s start with the basics and make a classic standard access list that denies R1’s loopback.

R2(config)#access-list 42 deny host 1.1.1.1
R2(config)#int f1/0
R2(config-if)#ip access-group 42 out

The loopback on R1 is blocked…

R1#ping 3.3.3.3 source 1.1.1.1
U.U.U
Success rate is 0 percent (0/5)

… but so is any other traffic that goes out of R2’s F1/0.

R1#ping 3.3.3.3 source F0/0
U.U.U
Success rate is 0 percent (0/5)

The first rule of Cisco’s ACLs is that there is an implicit deny (ip) all (all) rule at the end of every ACL. But this is not visible anywhere. You have to know it.

R2#sh access-lists
Standard IP access list 42
10 deny   1.1.1.1 (8 matches)
Extended IP access list BLOCK_HTTP

But if that ACL is empty? What if you apply an access list that does not contain any rules (was not declared)?

R2(config)#int f1/0
R2(config-if)#ip access-group 28 out
R2(config-if)#do sh access-lists
Standard IP access list 42
10 deny   1.1.1.1 (8 matches)
Extended IP access list BLOCK_HTTP

R1#ping 3.3.3.3 source 1.1.1.1

Type escape sequence to abort.
!!!!!
Success rate is 100 percent

Traffic passes. The inexistent ACL applied on an interface is ignored. But this is because you can’t have an empty classical (numbered) ACL. What if you do the same thing with a named ACL?

R2(config)#ip access-list standard EMPTY_ACL
R2(config-std-nacl)#exit
R2(config)#do sh ip access-list
Standard IP access list 42
10 deny   1.1.1.1 (8 matches)
Standard IP access list EMPTY_ACL
Extended IP access list BLOCK_HTTP
R2(config)#int f1/0
R2(config-if)#ip access-group EMPTY_ACL out

R1#ping 3.3.3.3 source 1.1.1.1

Type escape sequence to abort.
!!!!!
Success rate is 100 percent

Traffic is still not filtered. So, the rule is that a empty (inexistant or deleted)  ACL is ignored by the interface filter.

One more ACL applied on R2 with a deny all rule (no traffic should pass out of F1/0).

R2(config)#ip access-list standard DENY_ALL_ACL
R2(config-std-nacl)#deny any
R2(config-std-nacl)#do sh ip access
Standard IP access list 42
10 deny   1.1.1.1 (8 matches)
Standard IP access list DENY_ALL_ACL
10 deny   any (8 matches)
Standard IP access list EMPTY_ACL
10 deny   any (8 matches)
Extended IP access list BLOCK_HTTP
R2(config-std-nacl)#int f1/0
R2(config-if)#ip access-group DENY_ALL_ACL out

Ping form R1 is filtered.

R1#ping 3.3.3.3 source 1.1.1.1
Packet sent with a source address of 1.1.1.1
U.U.U
Success rate is 0 percent (0/5)

Since no traffic should go out the interface, a ping from R2 to R3 should also fail, yet it doesn’t.

R2#ping 3.3.3.3
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/20/44 ms

As a final rule, traffic generated by a router is never filtered by an ACL applied any interface of that router.

Scenario:
You have two routers running RIP, but the two routers aren’t directly connected because there is a third router between them. See topology below. How do you get routes across because RIP only communicates with routers that are directly connected?

The simple answer is to create a GRE tunnel between R1 and R3 so a tun interface simulates a direct connection of the two routers. But let’s take a more didactic approach to remember some things about RIP.

RIP v2 sends the updates to the address 224.0.0.9 that is a local multicast address (TTL=1).  But there is another, very important in some situations (like some Frame Relay networks), way to send routes, and that is via unicast to a statically configured neighbor. Configuration is done via the neighbor command in the router rip configuration.  The routes will be encapsulated in normal IP unicast packets and since RIP runs on top of UDP, they should be routed as any other packet.

R1:

interface Serial0/0/1
ip address 10.1.2.1 255.255.255.0
interface Loopback 0
ip address 192.168.0.1 255.255.255.0
router rip
version 2
passive-interface Loopback0
network 10.0.0.0
network 192.168.0.0
neighbor 10.2.3.3
no auto-summary

R3:

interface Serial0/0/1
ip address 10.2.3.3 255.255.255.0
interface Loopback 0
ip address 172.16.0.1 255.255.255.0
router rip
version 2
passive-interface Loopback0
network 10.0.0.0
network 172.16.0.0
neighbor 10.1.2.1
no auto-summary

You still need to have a network command for the interfaces when you send and receive the updates (in this case 10.0.0.0) otherwise the received updates will be ignored.

First thing you should be careful of is the fact that R1 and R3 need layer3 communication. So you do need static routes for the R1 and R3 routers through R2.

Having connectivity between each other, the router starts sending unicast packets with the routes. debug ip rip would show the following:

RIP: sending v2 update to 10.1.2.1 via Serial0/0/1 (10.2.3.3)
RIP: build update entries
172.16.0.0/24 via 0.0.0.0, metric 1, tag 0

Notice the update is sent to an unicast address and not 224.0.0.9.

Routes are received but they still are not in the routing tables. debug ip rip shows why:

RIP: ignored v2 update from bad source 10.2.3.3 on Serial0/0/1

This reminds us of how RIP works: if a router receives an update it checks to see if the source of the packet is on the same subnet as the IP configured on the interface. If they don’t match, the update is ignored. In our case, the source of the updates are not on the same network because R2 does not modify the packet source/destination in any way.

The solution to this is to disable the default mechanism with the no validate-update-source command in the router rip configuration. This way any updates will be accepted.

Here is a wanted route in the routing table of R3:

R    192.168.0.0/24 [120/1] via 10.1.2.1, 00:00:27

Notice that the next hop is not directly connected so it need to do a recursive lookup and use the static route to send it to R2 first.

S       10.1.2.1/32 [1/0] via 10.2.3.2

Topology:

Scenario:

The Host in the 192.168.0.0/24 network should get its IP address from a DHCP server.

Relay is the default router for the Host, but doesn’t have a DHCP service running. It will pass any DHCP requests from it’ f1/0 interface to the DHCP server that has DHCP pools configured on it, using the “ip helper-addres” command.

Between the DHCP router and the Relay router there is a public network, but behind Relay, there is a private network (Host is part of that network).  Relay will use NAT with overload (PAT) to service the private network.

Relay uses DHCP as it’s default route to the Internet, but DHCP doesn’t know about the private network in which Host is in (private networks shoudn’t be permitted to be accessed from the Internet).

Configurations:

DHCP:

ip dhcp pool DHCP_POOL
network 192.168.0.0 255.255.255.0
default-router 192.168.0.1

interface FastEthernet0/0
ip address 200.0.0.1 255.255.255.0

Relay:

interface FastEthernet0/0
ip address 200.0.0.100 255.255.255.0
ip nat outside

interface FastEthernet1/0
ip address 192.168.0.1 255.255.255.0
ip helper-address 200.0.0.1
ip nat inside

ip nat inside source list NAT_HOSTS interface FastEthernet0/0 overload

ip access-list standard NAT_HOSTS
permit 192.168.0.0 0.0.0.255

Host:

interface FastEthernet1/0
ip address dhcp

Problem:

Relay will receive a DHCP request (broadcast) on F1/0 interface. Because of the “ip helper-address“, Relay will transform the request from broadcast to unicast and send it to the DHCP router. The  DHCP request will reach the router, it will assign  an IP from the pool, but the reply will never reach Host.

Explenation:

Using “debug ip dhcp server events“, “debug ip dhcp server packet” and “debug ip packet“, we can find out the problem.

The first thing that could come to mind is the fapt that if Relay receives a packet on F1/0 interface (192.168.0.1) it will send an unicast message with the source IP address of that interface and a destination address of the ip-helper server. This is not true. The relayed request is considered to be generated by the local router (Relay). This means that the source IP address of the relayed request is that of the outgoing interface to the DHCP Server. Here is the debug ip packet output:

*Mar  1 02:33:23.127: IP: tableid=0, s=200.0.0.100 (FastEthernet0/0), d=200.0.0. 1 (FastEthernet0/0), routed via RIB

If  the source address of the IP packet does not have an IP address from the 192.168.0.0/24 network, how does the DHCP Server know from witch pool to give out a free address. The answer is a field in the DHCP protocol, called GIADDR (Gateway IP Address). The value of this field will be the IP address of the interface in the private network.

The problem is that after the DHCP server chooses an IP from the pool, it will reply to the unicast request, with another unicast packet that has the destination IP the GIADDR, not the source address of the request. The output from debug ip dhcp server events:

*Mar  1 03:13:33.719: DHCPD: DHCPDISCOVER received from client 0063.6973.636f.2d 63.6330.322e.3035.6230.2e30.3031.302d.4661.312f.30 through relay 192.168.0.1.
*Mar  1 03:13:33.731: DHCPD: Sending DHCPOFFER to client 0063.6973.636f.2d63.6330.322e.3035.6230.2e30.3031.302d.4661.312f.30 (192.168.0.2).
*Mar  1 03:13:33.731: DHCPD: unicasting BOOTREPLY for client cc02.05b0.0010 to relay 192.168.0.1

The DHCP router doesn’t know about the 192.168.0.0/24 network because that is a private network behind a NAT.

A solution to the situation is to add a static route on the DHCP router to the private network. But this would ruin the purpose of NAT. A better solution is to avoid the scenario by design (still, the situation could come up in lab environments and you should now know why it behaves the way it does)

ACL are usually configured for firewall configurations, for traffic filtering. When configuring ACLs, careful planing should be made so that in the moment when you are applying an ACL, things get filtered exactly the way you want it. In a lab environment tests can be made and if somethings doesn’t work right, you can start over. But in a live network router, filtering the wrong traffic could cause network outages.

If you are connected to the router via telnet or ssh (most likely in productions routers) it is very easy to lock yourself out of the router by denying the telnet or ssh traffic on an interface between you to that router. This is mostly because how IOS works. Any commands given in IOS are instantly commited to the live configuration. And, for example, if you make a configuration with an ACL and you forget about the implicit deny any (any) and you also forget to permit the telnet/ssh traffic, you might find yourself with the router not responding to any input after you apply the rules. It might take a while to figure out that you can’t access the router anymore and need to get physically to its location and either reload it or  use the console port to remove the ACL from the running-config.

One way of avoiding this is to schedule an automated reload in 10-15 minutes, while you are configuring, From enable mode issue the command:

#reload in MINUTES

This will reload the router after the specified number of minutes. It will ensure that if you lock yourself out, the router will revert back to the working startup-config. If the configuration was applied successfully, you can cancel the scheduled reload with the command

#reload cancel

Frame Relay is still very much a popular subject in exams, labs and in the real networks.

Any lab with topologies that run different protocols over FR must start with the layer 2 configuration of the Frame Relay switched network. FR Topologies like full mesh or hub and spoke require a Frame Relay Switch. A FR Switch is a normal router but specifically configured to do Frame Relay switching.

First of all, we need to tell the router to start switching Frame Relay traffic. From global configuration mode we need to issue the frame-relay switching command.

Then, on the interfaces to Frame Relay clients, we need to start sending keepalives (LMIs) by configuring the interface as DCE with the frame-relay intf-type dce command.

The last thing that the FR Switch needs to do is to route DLCI on the virtual cicuits. This is done to tell an interface where to put a received frame with a DLCI. The frame will be put on another interface with another DLCI.  The configuration is done per interface with the frame-relay route command. The command requires that you specify the incoming  DLCI, on which the switching decision will be maide, the outgoing interface, and the DLCI with which the frame will be sent (”freame-relay route IN_DLCI OUT_INT OUT_DLCI”).

If Inverse ARP is not disabled on the FR Switch, no DLCI-IP mappings will be required.

Topology:

Configuration:

R1(config)#int s0/0
R1(config-if)#no shut
R1(config-if)#encapsulation frame-relay
R1(config-if)#clock rate 128000
R1(config-if)#ip address 10.1.2.1 255.255.255.0

R2(config)#int s0/0
R2(config-if)#no shut
R2(config-if)#encapsulation frame-relay
R2(config-if)#clock rate 128000
R2(config-if)#ip address 10.1.2.2 255.255.255.0

FR-Sw(config)#frame-relay switching
FR-Sw(config)#int s0/1
FR-Sw(config-if)#no shut
FR-Sw(config-if)#clock rate 128000
FR-Sw(config-if)#encapsulation frame-relay
FR-Sw(config-if)#frame-relay intf-type dce
FR-Sw(config-if)#frame-relay route 102 interface s0/0 201
FR-Sw(config-if)#int s0/0
FR-Sw(config-if)#no shut
FR-Sw(config-if)#clock rate 128000
FR-Sw(config-if)#encapsulation frame-relay
FR-Sw(config-if)#frame-relay intf-type dce
FR-Sw(config-if)#frame-relay route 201 interface s0/1 102

Running Configurations:

R1:

interface Serial0/0
ip address 10.1.2.1 255.255.255.0
encapsulation frame-relay
clockrate 128000
no fair-queue
end

R2:

interface Serial0/0
ip address 10.1.2.2 255.255.255.0
encapsulation frame-relay
clockrate 128000
no fair-queue
end

FR-Sw:

interface Serial0/0
no ip address
encapsulation frame-relay
no fair-queue
frame-relay intf-type dce
frame-relay route 201 interface Serial0/1 102
end

interface Serial0/1
no ip address
encapsulation frame-relay
frame-relay intf-type dce
frame-relay route 102 interface Serial0/0 201
end

This is the simplest use of a Frame Relay encapsulation and it’s between two routers, without a Frame Relay Switch. PPP or HDLC would make more sense to use in these types of links, but it is useful in labs.

In a back-to-back scenario is important to remember what the FR Switch should be doing: being the DCE and sending the keepalives to maintain the layer 2 link to the client router. Because of the fact that no FR Switch is present, the lack of keepalives being sent must be ignored using the “no keepalive” command. Also, Inverse ARP won’t work, so manual IP-DLCI mapping will be needed.

The FR Switch should be the one doing swapping of DLCIs on the network so the frames arrive at their destination with the correctly mapped DLCIs. In this case, we will need to have the same DLCI set in the manual mapping so the routers match entries in the mappings.

The topology:

Configuration:

R1(config)#int s0/1
R1(config-if)#no shut
R1(config-if)#clock rate 128000
R1(config-if)#ip address 10.1.2.1 255.255.255.0
R1(config-if)#encapsulation frame-relay
R1(config-if)#frame-relay map ip 10.1.2.2 42
R1(config-if)#no keepalive

R2(config)#int s0/1
R2(config-if)#no shut
R2(config-if)#clock rate 128000
R2(config-if)#ip address 10.1.2.2 255.255.255.0
R2(config-if)#encapsulation frame-relay
R2(config-if)#frame-relay map ip 10.1.2.1 42
R2(config-if)#no keepalive

Running configurations:

R1:

interface Serial0/1
ip address 10.1.2.1 255.255.255.0
encapsulation frame-relay
no keepalive
clockrate 128000
frame-relay map ip 10.1.2.2 42
end

R2:

interface Serial0/1
ip address 10.1.2.2 255.255.255.0
encapsulation frame-relay
no keepalive
frame-relay map ip 10.1.2.1 42
end

One of the things that make Command Line Interfaces, like Bash, very efficient for administration is the output manipulation with piping and redirecting. Cisco IOS has most of the Bash equivalent modifiers, and administrators that know how to work with them can do things much more faster… this can make the difference in a lab exam or in the real world. Most show commands support this features and depending on the IOS, you have more or less features.

The usual “show run” command prints a large output, from which you need only a few lines. You can only scroll down with space and enter (the the Linux more command). If you are searching for a keyword in the running config, you can go to the line that contains the string using the slash key, like in vim or more or less in Linux. So, “/KEYWORD” after running the show command, while scrolling, will take you to the wanted line.

If you want from the output just some lines, you can filter them, just like piping the output to grep in Linux. You can use the ” | ” after the show command to see how you can filter (be careful, there is a space before and after the |). To print just the lines that have a keywork, use “ | include KEYWORD“, and to print all lines except the ones what have the keyword, use “ | exclude KEYWORD“. If you want to print out all output starting with a line that contains a keyword until the end of the lines, use “ | begin KEYWORD“.

Taking advantage of the hierarchical structure of the running config, you can print out just a section of the output. For example, “show run | section   router ospf 1” will list the configuration for the OSPF process 1 and “show run | section interface Serial0/0” will print the configuration for the specified interface. Be careful, this is case sensitive and you need to mach the case of the line in the running config (”Serial 0/0″ will work, “serial 0/0″ won’t).

Redirection into a file is also possible. “show run | redirect flash:run” will put the contents of the running config into a file called ‘run’ in flash memory. This is similar to the “>” operand in Bash. Using redirect, the content of the target file will be replaced. You can append to the file (like “>>” in Bash) with “ | append FILE“.  “ | tee FILE” works like redirect, but it also prints the output to the screen.

Regular expressions are also supported. If you like to print from the routing table, the routes received from RIP, you can filter with “show ip route | include R” and the routes from EIGRP with  “show ip route | include D”. But you can do this in one line, filtering with both conditions, with “show ip route | include [RD]“.

Slightly off topic, but good to know, is how to stop output. For example, traceroute to an unreachable location, will try 30 hops before it stops, and this might take a long time. To break the action hit the key combination “Ctrl+Shift+6“.