HSRP Multicast query

[Nicholas Beard]

Hi all,

I have a query regarding HSRP and their associated multicast messages.  I have two Cisco 4948E switches in a distribution layer connected via a 20Gbps link at Layer 2 and two Cisco ME routers provided by an ISP running HSRP connected at Layer 3.

Physical Connectivity is as follows -

ISP Router 1 LAN interface connected to Switch 1

ISP Router 2 LAN interface connected to Switch 2

The ISP routers are configured for HSRP as follows -

ISP Router 1 LAN interface - 1.1.1.2/24

ISP Router 2 LAN interface - 1.1.1.3/24

Virtual Address - 1.1.1.1/24

The Cisco 4948E switches are configured as follows -

Switch 1 ISP Router 1 interface - 1.1.1.5

Switch 2 ISP Router 2 interface - 1.1.1.6

The Cisco 4948E switches have dual ISPs and perform basic load balancing using default routes.  I am also using provider aggregate addresses to subnet into smaller segments to provide to different areas of the network.  Each 4848E switch has a default route to the HSRP virtual address of the ISP routers as follows -

ISP 1

ip route 0.0.0.0 0.0.0.0 1.1.1.1

ISP 2

ip route 0.0.0.0 0.0.0.0 2.2.2.2

From a routing perspective everything is working fine with no problems, and we will soon be introducing BGP inplace of the default static routes once we obtain provider independent status.

The issue is with the HSRP hello messages and their status'.  The ISP routers are both showing the following -

Active router is local

Standby router is unknown

This is due to the hello messages never reaching other.  My understaing of this is the HSRP multicast messages are sent to 224.0.0.102 at layer 2 and because I am using layer 3 ports from the 4948E switches the messages are not being exchanged as the layer 3 ports on my switches are not forwarding the multicast.  Is there any method i can use to forward these multicast messages between the Switches and ISP routers in order to achieve a corrrect HSRP status?

If at all possible I would like to avoid the requirement for adding another layer 2 switch into the scenario. 

Thanks

Nick

[Nicholas Beard]

I think the best solution here would be to change the layer 3 interfaces on the switches to a layer 2 access port in a specific "external" facing VLAN.  Similar to the following -

Switchport mode access

switchport access vlan 100

Add the VLAN to the layer 2 trunk between switches and create a VLAN 100 SVI interface in the /29 network range containing the ISP virtual IP address similar to the following -

Switch 1

int vlan 100

ip address 1.1.1.5 255.255.255.248

Switch 2

int vlan 100

ip address 1.1.1.6 255.255.255.248

This should ensure the HSRP messages can be exchanged and the switches can still route to the ISP.

[Raju Sekharan]

Hi Nick

HSRP Active and standby routers interfaces should be connected via Layer 2 path. These are not routable packets and send with TTL=1

I have a query. Why do you want to give IP address for the Switch LAN interfaces which connects to HSRP router?

Thanks

RAju