[c-nsp] VSS1440 to ASR1002 - MEC issues

Alasdair McWilliam alasdairm at gmail.com
Fri May 1 11:29:56 EDT 2009


I'm currently deploying two Cisco 6509-E chassis with VS-Sup720-10GE  
(in a VSS 1440 cluster/configuration) with dual ASR 1002 routers to  
provide aggregation of multiple upstream links (running multiple BGP  
and EIGRP sessions).

I wanted to utilize MEC between each ASR and each 6509 chassis to  
build in as much resilience as possible. However this configuration  
seems to be playing up and so I thought I'd ask the experts!

Physical Topology:

ASR Gi0/0/0 into 6509 Chassis 1 Module 1 Port 1
ASR Gi0/1/0 into 6509 Chassis 2 Module 1 Port 1

The ASR is running IOS-XE 2.3.0 (IOS 12.2(33)XNC) AISK9 with dual IOS  
The VSS chassis are running IOS 12.2(33)SXI1 ISK9 with a 4x 10GE VSL  
(2 supervisor 10GE interfaces, 2 10GE interfaces on a 6708-10GE line  
I'm just using CAT6 between the ASR and the 6748-GE-TX line cards in  
the VSS boxes.

ASR configuration:

interface Port-Channel1
ip address x.x.x.5
ip hello-interval eigrp 100 2
ip hold-time eigrp 100 6
ip authentication mode eigrp 100 md5
ip authentication key-chian eigrp 100 vcoresw1-chain
ip summary-address eigrp 100 255
no ip redirects
no ip unreachables
no ip proxy-arp
no shut

interface Gi0/0/0
channel-group 1
no shut

interface Gi0/1/0
channel-group 1
no shut

Cisco VSS configuration:

int Gi1/1/1
no switchport
channel-group 3 mode on

int Gi2/1/1
no switchport
channel-group 3 mode on

int Po3
desc *** MEC to br1-po1 ***
no ip redirects
no ip unreachables
no ip proxy-arp
ip vrf forwarding edge-vrf
ip address x.x.x.6
ip hello-interval eigrp 100 2
ip hold-time eigrp 100 6
ip authentication mode eigrp 100 md5
ip authentication key-chain eigrp 100 br1-chain
no shut

The problem I am experiencing seems to be one way traffic between the  
VSS cluster and the Border Router. Pinging across this /30 subnet does  
not work in either direction. EIGRP relationships build when the Po  
interfaces first come online and then immediately time out moments  
later. The VSS cluster then does not see any further EIGRP traffic  
from the ASR. However the ASR seems to think it's successfully  
building an adjacency to the VSS. However this times out due to 'retry  
limit exceeded' every minute or so, but seems to think it re- 
establishes again.

This problem persists if we drop the PortChannel to just one Gigabit  
Ethernet interface. The second interface can be shut down or actually  
removed from the Po config (eg. no channel-group 1).

The really interesting thing is, with one link, if we remove the  
channel-group comand from the one remaining ASR interface, all of a  
sudden the link springs to life. Pings between the ASR Gi0/0/0  
interface and the Po3 VSS interface are successful. EIGRP relationship  
comes up immediately and is stable, and routes are exchanged as you'd  

How does this work? With the ASR thinking it's a non-etherchannel  
interface, but the VSS thinking it IS an EtherChannel (with 1 member),  
surely it should just fail?

Am I doing something wrong or could this be a bug in either VSS or the  

It's not earth shattering, we could just configure 2 EIGRP sessions  
between the VSS and the ASR (4 in total with 2 ASRs) but don't think  
this is as clean an implementation as MEC across fully redundant  
chassis and line cards (one of the big selling points of the VSS !!)

Any help would be much appreciated!


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