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New Member

IP CACHE ISSUE(fast switching)

I have 3 networking devices connected on LAN , one is 3641 , other is also 3642 & third one is RAS, now i have added a specific route in router 1 i.e 10.10.1.4/30 pointing to the ethernet port of router 2 , both ethernet port are running fast switching , but when i check the show ip cache output it is pointing the block of 10.10.1.4/30 pointing towards the RAS , where as when i check show ip route , it shows me the correct route, but i cant ping 10.10.1.5 or 10.10.1.6 from the router 1 , since all the packets for the destination is getting forwarded to ethernet port of RAS, so there is a descrepency in the show ip cache output & show ip route output i have tired to clear arp, clear ip cache, but doesnt help.......if some one can help me on this issue.

9 REPLIES
New Member

Re: IP CACHE ISSUE(fast switching)

First the most important thing is you can't ping the aforementioned devices.. possibly a routing issue. Could you copy and paste your configs of the three devices in a reply. Check your ip addresses masks set on all 3 devices;you are using a /30 which has only two valid hosts for network 10.10.1.4... 1.4 is the network id, .1.5, and 1.6 are valid hosts, and 1.7 is an all host-broadcast. 1.8 is your next network id. Change your mask for this network or change the ip and mask of the RAS to 10.10.1.9 /30. Then put in applicable ip route in router 1 pointing to the new network the RAS is on and the RAS's gateway should be router 2(10.10.1.6) IA debug ip packet before and after your config changes should show where your ICMP packets are dropping and if it successfully following the correct path. For addition helps check out CCIE TCP/IP Routing Chapter 3: Static Routes: Troubleshooting Static Routes p114-116. . I hope this helps

l-mathews/CCNP

acmiorelli/CCNA

NYMANOC-Cablevision

New Member

Re: IP CACHE ISSUE(fast switching)

Hi

The diag is as follows :

R1(172.16.1.1/24)--------SWITCH--------R2(172.16.1.2/24), also connected is RAS with the ip address(172.16.1.3/24), now the 10.10.1.5 is the wan ip address in R2, the default route in R2 is pointing towards R1, now it is a simple case of ping which i am trying from R1, after debugging the ip packet detail i get the following o/p

1w3d: IP: s=172.16.1.1 (local), d=10.10.1.5 (Ethernet0/0), len 100, sending

1w3d: IP: s=172.16.1.1 (Ethernet0/0), d=10.10.1.5 (Ethernet0/0), len 100, rcv

d local pkt.

The packet i belive are getting looped in the ethernet port itself, the arp entries contain all the details of the 172.16.1.2

The routes i have checked up are correct .

Pls advice

New Member

Re: IP CACHE ISSUE(fast switching)

If you can copy an paste your network IP configs as a whole.. including all devices and interface, networks and subnets

What kind of switch is that between R1 and R2. In addtion your IP debug packet suggests that the ICMP Packet is being sent to and received by 10.10.1.5 but there is no reply back from 10.10.1.5 meaning 10.10.1.5 does not how to reach 172.16.1.1Have you done a trace from 172.16.1.1 to 10.10.1.5 and 1.6 and examine the debug ip packet. If you thing the packet is being looped on the ethernet port.. maybe the problem is the switch between R1 and R2. One question.. if it is a switch.. why are you segmenting r1 and r2 if they are not on different networks or subnets. In addition whose IP address is 10.10.1.6 is that RAS and is it ethernet as well..

l-mathews CCNP

nymanoc-cablevision

New Member

Re: IP CACHE ISSUE(fast switching)

What Routing protocol are you running.. please copy and paste your route statments including the default route.for r1 and r2 in addition to the arp entries of the the RAS Server. Is R2 setup with any passive interfaces. I cannot stress enough to attempt a trace from RAS Server to R1 and Vice Versa.. also from R2 to R1. Then examine the output of your debug ip packet. Please copy and paste in reply.. I hope this helps.

l-mathews/ CCNP

nymanoc-cablevision.com

Re: IP CACHE ISSUE(fast switching)

Some things to try:

Disable fast switching, 'no ip route-cache' on the interfaces and see if this solves the problem if you haven't done so already.

Remove the switch from the picture if possible and connect routers back-to-back with crossover to eliminate a issue with the switch. If it works this way, then you can focus on finding out whats going with the switch. Can you ping other devices attached to the switch?

The switch shouldn't be sending packets back toward R1 unless its MAC address table (CAM) is messed up. What type of switch is this?

New Member

Re: IP CACHE ISSUE(fast switching)

I have tried by diabling the route-cache feature , in fact tried with cef also , but still the descrepency is there between the sh ip route o/p & the show ip cache or show ip cef.

The switch is a cisco switch 2924.

There are other routers & networking devices connected so i can't connect a cross a cable between the two.

if ARP is messed , by clearing arp it shd either get restored or should give o/p has incomplete if arp is not able to find the MAC address of the R2, but that is not the case.

New Member

Re: IP CACHE ISSUE(fast switching)

Is The RAS Server hanging off the switch.. do a trace from R1 to R2 and then to RAS.. before you do this turning off fast switching and enable process switching on R1 and R2 and then do the above trace and examine the output of debug ip packet. Fast Switching packets are not displayed only process-switched packets. If the packets are getting looped examine the arp table for an entry for your RAS. Turnoff proxy-arp on R1. you should also examine all your routing statements includind default routes on R2.. It seems that the ICMP echo packets are reaching its destination but the ICMP echo reply packets are not getting back. please copy and paste your routing statements from R1 and R2. to make sure packets know how to correctly reach R1. Case Studies similiar to this problem are documented in CCIE Routing TCP/IP Volume 1 Chapter 3 static Routing P 110-125.

I hope this helps.

l-mathews, CCNP

nymanoc-cablevision

New Member

Re: IP CACHE ISSUE(fast switching)

You may try to do an extended ping from R2 to R1.. specifying a source of the wan ip address 10.10.1.5.. examine the output and see if R2 can reach R1..Also try an extended ping from R2 to the RAS, specify different sources (each interfaces on R2 .. first the 172.16.1.2 /24 and 10.10.1.5.. examine the output it will help narrow down where exactly the problem is and where the packets are getting looped or dropped. If ping fails.. you should try a trace to see where on the link the packets are getting looped or dropped.

l-mathews, CCNP

nymanoc-cablevision

Cisco Employee

Re: IP CACHE ISSUE(fast switching)

could you get a 'sh ip route 10.10.1.5' and 'sh run int eth 0/0' a 'sh ip int ether 0/0'.

Then get a 'sh ip arp x.x.x.x' where x.x.x.x is the next-hop for 10.10.1.5 mentioned in the 'sh ip route'.

On the switch do a 'sh cam xx-xx-xx-xx-xx-xx' where xx-xx-xx-xx-xx-xx is the mac address given above.

What software version are you running on R1 ?

Thanks,

Gilles.

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