SHOW MAC ADDRESS OF INTERFACE CISCO DRIVER
SHOW MAC ADDRESS OF INTERFACE CISCO MANUAL

On one side, under the 元 interface use mac-address H.H.H.H.

We can use the formula to check this:īut what are the options for fixing the problem where the MAC addresses are the same on both sides? Therefore the virtual MAC address on the 元 interfaces were both 28. When I checked both switches I found they both had a switch virtual domain ID of 10. The last two bits of the first octet is allocated for protocol mac-address which is derived by adding the protocol ID (0 to 3) to the router MAC address. The MAC address range reserved for the VSS is derived from a reserved pool of addresses with the domain ID encoded in the leading 6 bits of the last octet and trailing 2 bits of the previous octet of the mac-address. It’s used in the formula to calculate this pool. But the switch virtual domain ID is significant in determining the virtual MAC address pool. This ensures the same MAC address pool is used at all times. Hence Cisco recommend using mac-address use-virtual under the switch virtual domain ID.

SHOW MAC ADDRESS OF INTERFACE CISCO MANUAL

This shouldn’t be a huge deal but if there are any other devices out there that are ignoring gratuitous ARP they will require manual intervention to get them working which will cause further service disruption. But if the entire VSS is reloaded and the other switch happens to come up first the MAC address pool will change. This MAC address pool is maintained by VSS when one (and only one) switch is reloaded. When when the first switch comes up, VSS uses the MAC address pool from that member and uses that pool across all 元 interfaces. I’ll explain why Cisco recommend this option. Cisco recommend that you enable virtual MAC addresses ( mac-address use-virtual) under the switch virtual domain. One of the first things you do when configuring VSS so set the switch virtual domain ID. How could this happen?Īs mentioned in the first sentence both ends had a Cisco 6500 Catalyst switches running VSS. Checking the MAC address for the interface the L2 circuit was connected to at site A and the MAC address for the interface the L2 circuit was connected to at site B showed the same MAC. Bingo! The issue was at Layer 2 as I could see ARP was incomplete on both sides for the neighbor addresses. Troubleshooting started and as always you start at the lowest OSI layer and work up. But as soon as they were connected the neighbors started flapping. The interfaces they connected to were configured as 元 and EIGRP was run between the two sites to share routes. At both ends were Cisco 6500 Catalyst switches running VSS. *ARP information is cached by TCP/IP hosts, so the information may be cached locally and in the case of the Default Gateway's MAC address, it almost certainly will be almost immediately after the host powered on.The other weekend I connected a L2 circuit between two sites. If the egress interface is an Ethernet interface, the whole ARP/encap process happens again. Once the frame arrives at the gateway, the Ethernet interface receives and decapsulates (destroys) the ETH frame, passes the IP packet up to the IP process running on the router to determine the egress interface and the next destination - either the ultimate destination or the next hop router. In this packet, the DA of the IP Packet will remain the address of the ultimate destination, but the DA of the ETH frame will be the MAC of the DG/SVI. The IP packet and the destination MAC address is passed to the Ethernet driver/code to create the ethernet frame and encapsulate the IP packet. In this case, the host ARPs* for the MAC address of its Default Gateway - the SVI.

However, when those 24 bits are not identical, the destination is remote, so the host must send the packet to its gateway for routing.

SHOW MAC ADDRESS OF INTERFACE CISCO DRIVER

MAC is passed to the Ethernet driver to encapsulate the ETH frame with the Destination Address (DA) of the IP packet and the ETH frame being that of the destination host. Once the MAC is returned, the IP packet and Dest. ARP sends out a broadcast that includes the destination IP address and asks for a corresponding MAC address. In which case, the host would need to discover the MAC address of the destination using the ARP* protocol. It then compares these 24 bits to the first 24 bits of the destination, if each bit is the same, then the destination is local. For instance, a subnet mask of 255.255.255.0, also known as a /24, means that the first (left-most or high order) 24 bits of the host's IP address are its network ID. This process involves the host determining what portion of its own IP address is its network ID based on its subnet mask.

TCP/IP hosts determine if a destination is local or remote through a process called ANDing.

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